Executive Summary

GPS Satellite Constellation

This page provides a brief history and introduction to GPS (Global Positioning System) and the science behind it's ability to calculate various parameters on the earths surface. The GPS system, which was originally developed for defense purposes, has recently been utilized for a wide array of uses such as recreational activities and business purposes.

The science behind the GPS and its ability to calculate the distance from each satellite to the GPS receiver is done using trilateration and complex time keeping techniques with the use of atomic clocks.

The "Earthmate GPS PN-20" was disassembled, examined, and reassembled by the group as part of our study of the GPS. The step by step disassembly and examination is documented with various pictures to illustrate the procedure. All the components names and features are described with the corresponding functions.

Field experiments with the GPS receiver were conducted by taking elevation measurements using the GPS and comparing them to known benchmark elevations from differential leveling techniques. Results indicate that the GPS receiver is inaccurate for elevation measurements and is best used in open areas with a clear view of the sky.

Conclusion to the GPS research project covers Group 12's experience with the Wiki page and the research behind the GPS calculation methods, functions, components and overall accuracy. Further conclusions cover the attitudes and recommendations of the group members.

Background: History of GPS

GPS began as a military research project in the 1960s and 1970s. It was originally planned as a US military operation that could be used for navigation and time-telling; however, it was decided that the GPS system would be opened to civilian use before the system was even completed.

This was done in response to an incident in 1983, when a U.S. passenger aircraft unintentionally entered Soviet airspace and was shot down, killing all 269 passengers on board. It was accepted that access to better navigation tools could prevent such occurrences in the future, so then President Ronald Reagan issued a directive guaranteeing that GPS signals would be available at no charge to the world when the system became operational.

The GPS (Global Positioning System) was created on June, 26 1993 with the addition of the 24th satellite launched into space; however, the system became fully operational in 1995, with a signal for military users and a less-accurate signal for civilians. Since that time, the GPS system has been accessible to an unlimited number of global military, civilian, and commercial users. In 2004, President George W. Bush issued an updated policy that maintained this policy, and keeps civilian GPS free of direct user fees.

When the GPS system was created, the Defense Department inserted timing errors into its transmissions to limit the accuracy of nonmilitary GPS receivers to 100 meters; however, this “selective availability” was eliminated in May 2000.

The GPS system is a network of 24 satellites that transmit high-frequency, low-power radio signals to a GPS receiver that allows the receiver to determine information such as its location, elevation, velocity and time. GPS receivers rely on a clear view of the sky for accurate data. They are best used while outdoors and away from tall buildings and heavily wooded areas.

Background: Science of GPS

The GPS system is composed of three technological components. The main component being an array of at least 24 GPS satellites, in one of six medium earth orbits. The satellites circle the earth every 12 hours emitting continuous navigation signals. The other two components main GPS network operation stations, and the end user devices. The Ground stations precisely track each satellite's orbit, and are the interface by which the U.S. military keeps the GPS system running. The end user devices are the devices that allow any military, commercial, or private entity to utilize the GPS signal to give meaningful data.

GPS Satellite Movement

The GPS system is used to provide precise data about location, time, and movement. In order to do this, the system must be able to solve for 4 variables – x, y, z, and t. The x, y, and z variables represent a three-dimensional coordinate system, and thus give information about location. The t variable is time, and is used by the user in collaboration with ongoing communication with the network, in order to have information about movement. Movement information consists of allowing users to see their path of travel, as well as their velocity. The time variable is also necessary for the system to be able to determine the other three variables of location. The scope of distance between earth and the GPS satellites, as well as the speed at which the orbiting GPS satellites are traveling makes it necessary to have an incredibly accurate measurement of time in order to determine location. This is done by analyzing the microwave signal received by the end-user GPS device.

The small differences in the time it takes for the signals of different satellites to reach the device are used to calculate the distance from the satellite and its three dimensional location. This three dimensional location is found with a set of imaginary spheres of know area located around the center of each satellite. Three spheres are required for two dimensions and four spheres for three dimensions.

Example of 2D position determination with 3 satellites

This is because there are four variables to solve for (x, y, z, and t); therefore, there must be a minimum of 4 satellites in contact with a GPS device. While the GPS device needs 4 satellites in order to function, there are far more than 4 satellites that a GPS device can receive data from at any one time. It depends on the location of the device, as well as the location of the satellites in orbit, but there are usually between 8-10 satellites in contact with a device at any one time. The image to the right shows how the line of sight between a GPS device and the GPS satellites changes with time. This allows for greater reliability and some greater accuracy in the system.

To explain the process of locating an object within a set of spheres, image a 2-D plane in which a GPS device is finding its location using three imaginary spheres created by three different satellites around it. The common intersection of the three spheres gives the location of the GPS unit. This method of determining the location of the GPS unit based on spheres of known area is called trilateration.

The calculated distance from the satellite to the GPS begins with the transmission of a long pseudo-random code sent from the satellite to the GPS. At the same time the GPS also runs the same code sending it to the satellite. The signal sent from the satellite will lag behind the transmitted signal sent from the GPS to the satellite. This lag in time is equivalent to the travel time of the signal and is multiplied by the speed of light to determine the distance the signal has traveled.

The timing of these radio signals from satellite to receiver and vise versa are only accurate if the clocks within the satellite and receiver are synchronized to the nanosecond. Each satellite uses an atomic clock to keep time; however, due to the price of atomic clocks the GPS receiver utilizes a conventional quartz type clock which is less accurate than an atomic clock. Due to the fact that the GPS receivers do not utilize atomic clocks a great deal of inaccuracy of the exact location is existent. Errors in the internal clock of the GPS require that the spheres be adjusted each in an equal amount until the intersection becomes defined. This method often gives inaccuracies of 10 to 20 meters. The GPS receiver remains synchronized with the satellite by analyzing the incoming signals from four satellites to determine its own inaccuracies. The correct time to use will exist only from the atomic clocks installed within the satellites. Since only one correct time exists the signals will align to a single point in space for the receiver to obtain. The GPS receiver will then set its time to the signals received from the satellites. The GPS receivers will continuously reset the quartz clock time to the atomic time from the satellite as long as the receiver is on.

The resulting distance calculated from the accurate time keeping between the satellites and GPS are used in conjunction with the actual location of the satellites. In order for the GPS receiver to determine the location of the satellites it utilizes an installed almanac which tells the receiver exactly were the satellite is in orbit at any given time. This information is easily obtained because the satellites travel in predictable orbits. The known locations of the 24 satellites are located in six orbital planes at an altitude of 20,200 Km.

Before Disassembly

Eartmate PN-20 Specification and features

The PN-20 comes with the several components and features that make it an all-in-one device indeed. The PN-20 includes a DeLorme Topo USA 7.0 DVD software with full, updated U.S. topographic and street maps that can be exported as needed to the device. The PN-20 also has on-device highway-level world base map, as well as discs that contain pre-cut map packages of the entire United States that can be transferred onto the SD card for insertion directly into the PN-20. Other components of the PN-20 are 1-GB SD Card & Reader, USB interface cable, baterries, and a comprehensive user's manual with usage scenario tips.

The PN-20 is waterproof standard, has impact-resistant rubberized housing, a bright color screen,measures 2.43″ W x 5.25″ H x 1.5″ D, and weighs 5.12 ounces. It has buttons that provides the user with access to all of the functionality on the device. It features a STMicroelectronics chipset with SiGE RF front-end and DeLorme firmware for outstanding signal acquisition and retention and is said to work equally well under dense foliage or in-vehicle. It also has 75MB of internal flash memory in addition to the preloaded world base map and can provide multiple views of the same GPS location.

Basic Functions of theEarthmate GPS PN-20 Device

Button

Function

Power

The POWER button is used to turn the Earthmate on and off, to change the backlight settings, or to reset the device. It is located at the bottom-right corner of the button area.

In/Out

The IN and OUT button allows the user to zoom in or zoom out on a map.

Page

The PAGE button allows the user scroll through all of the enabled “Pages.”

Menu

The MENU button allows the user to navigate through a multitude of functionalities available in the PN-20.

Find

The FIND button, labeled with a magnifying glass, allows the user to search for a waypoint, address, coordinate, and etc, based on its name or its proximity to the current map center.

Mark Waypoint

The Mark button, labeled with a push pin, is used to mark a waypoint at one's current GPS location.

Arrow Keypad

The ARROW keypad enables the user to move the map cursor left, right, up, or down, pan the map by moving the cursor on the edge of the map, highlight options in menus or lists, and highlight characters in the keyboard screen.

Enter

The ENTER button is used to select a menu entry or on-screen button/field and to get information about a point on the map.

Quit

The QUIT button is used to cancel any action or reverse the page sequence.

Disassembly, Assembly, and Component Description

The disassembly process was pretty easy overall. We were able to open the device using a small "phillips" screwdriver. The only minor difficulty was handling the screws, because they are really tiny and can be lost if care is not taken. Also, we needed to pay close attention to the rubber joint when opening or closing the device in order not to put it off track as putting it off track could affect the impermeability negatively and therefore quality of the device.
Below is the procedure we went through to take the device apart, explore the different hardware components, and to put it safely back together.

Step

Description

Picture

1

The back cover was taken off by loosening the screws on the back of the device that hold the cover in place. This process simply required the twisting of the screws by hand. The back cover, which is made of both plastic and rubber, protects the batteries as well as the SD card. The picture shows the batteries' placement. The PN-20 works on two AA batteries or R. CR-V3 Li-Ion approved rechargeable batteries manufactured by Moyoto Industrial Co, Ltd. AA Energizer batteries can last for a maximum of about 14 hours in this device.

.

2

Next, the batteries were taken out and the SD card slot lifted up. The picture shows the 1GB SD card partially inserted into the slot. Pre-cut map packages can be transferred onto the SD card for insertion directly into the PN-20. The the memory size of the SD card enables is to accommodate large files including the Aerial Data Packets and topographic maps.

3

These pictures show the USB attachment and the slot on the back of the PN-20 into which it fits. The USB attachment enables the PN-20 to be hooked up a computer allowing you to create, save, and transfer detailed GPS-accurate information between the unit and the computer. You are able to prepare maps and imagery on the desktop, including waypoints, tracks, and automatically generated routes and then transfer this information to the PN-20 via the USB connection. You are also able, once you’ve been in the field or in-vehicle with the PN-20, to transfer your new and updated waypoints and track logs back to the Topo USA desktop software,edit, and save these files as needed.

4

In this step the plastic back cover was unscrewed using a small screwdriver. A total of ten small screws held the back cover in place. The back cover protects and helps to keep the circuit board and the other internal components in place. A rubber washer that is located in-between the front and back covers ensures that the device is impermeable to water and other fluids that will damage the internal workings of the PN-20.

The different color wires connect the device to the batteries to generate the power required to work the PN-20.

5

Here the back cover containing the antenna receiver and the emergency battery storage is completely separated from the front part containing the circuit board and the display screen. The antenna connects to the receiver and transmitter and allows the GPS receiver to obtain strong data signals from various satellites around the globe. The information received by the receiver is sent to the circuit board which processes the information and displays it on the screen. Information is also transmitted from the SD card to the screen and vice versa via the circuit board.

6

This picture gives a closer look at the circuit board and all the micro chips the are involved in the processing and transmitting of data between the satellites, the screen, the SD card, and the computer the PN-20 may be hooked up to.

7

At this stage the PN-20 is being reassembled. The wires from the power source and those from the antenna receiver were connected back appropriately.

8

Finally, the rubber washer that ensures impermeability was replaced and the two plastic back covers were screwed back in place. The PN-20 worked perfectly after the dissection process.

Receiver Systems

GPS Receiver Block Diagram

The basic receiver systems include:

1. Front End - the GPS L1 signals are received at the antenna and amplified by the Low-Noise-Amplifier. The radio frequency front-end further filters, mixes, and amplifies automatic gain control signal down to the intermediate frequency where it is digitally sampled by a Analog-to-Digital converter.

2. Baseband Processor/CPU - the Analog-to-Digital conversion samples of GPS Course/Acquisition code signals are correlated by the digital signal processor and then formulated to make range measurements to the GPS satellites. The digital signal processor is interfaced with a general-purpose CPU, which handles tracking channels and controls user interfaces.

3. Memory - the processor runs applications stored in memory. The operating system is stored in non-volatile memory such as EE/FLASH/ROM.

6. Power Conversion - converts input power from battery source to run various functional blocks.

Field Experiment

Our group felt it was important to experiment with the GPS device to determine the practicality of its use in certain situations. We decided to test the device's ability to accurately determine elevations. Earlier in the term, our lab group performed a Differential Leveling experiment to determine the elevation of the first step of the statue outside of the DAC. We attempted to re-do this experiment, using the GPS device to record the elevations of the Benchmark, Turning points, and Statue. The GPS readings were recorded and compared to our lab data to determine the accuracy of the GPS.

After putting the GPS device together and pressing some buttons we had it up and running fairly quickly. After walking around a bit we found it was very easy to get some elevation reading from different pages in the GPS. Also we discovered that further inspection into the options can gave an error reading for each spot. The error readings seemed to level out after standing still for a few seconds.

There were a few interesting things to point out with the GPS unit. We started between buildings it took a while to connect to the satellites.Once connected there was a great deal of error. We took three or four readings and then decided to go back and double check what we did. The results changed greatly between the same spots just depending on where we approached from. After we went back and saw there was great error we decided to go up and record the error at each spot with an elevation. Our results below show that while walking in an obviously uphill direction the GPS device kept giving us lower numbers as we were walking downhill. We saw this as a big error in the system. The GPS elevation measurements were different from our lab measurements from the beginning. Before attempting the Differential Leveling lab, we were given a benchmark elevation of 202 feet for man-hole 9. Using the GPS device on the exact same spot gave us a starting elevation of 113 feet +or- 38 feet. Even the highest elevation is more than 50 feet below the given benchmark. As we moved toward the DAC statue, we saw that the elevation readings were consistantly lower than the measurements we determined from the Differential Leveling lab. This could be an affect of using a different base elevation to determine the measurements. We also noticed that the measurements taken in between the buildings were drastically different from those taken near the street. This was probably caused by the buildings blocking some of the satellites from connecting with the device.

After doing this small experiment we determined the GPS would be ineffective in determining an accurate reading for a detailed survey area. If you had to be very accurate in a starting location for a building project of some kind this would not be a good method to use. However with the software involved and other capabilities it may be useful as a preliminary survey device to get an overall feel of the site you are working on.

Experimental Data

Experimental Data Collected

Station

Differential Leveling Data

GPS Readings

MH-9

202.2 ft

113ft +/- 38ft

TP-1

210.71ft

169.46ft +/- 46ft

TP-2

216.38ft

70ft +/- 42ft

TP-3

216.63

42ft +/- 28ft

Statue

225.29ft

57ft +/- 34ft

After analyzing the results of the above experiment, we determined that the GPS device would not be accurate enough for important elevation readings. However, I have personally seen contractors use GPS devices to verify the accuracy of the base elevations of building pads. Therefore, GPS devices can be used when operated in very open areas and when high quality equipment is used.

The GPS experiment and data interpretation was performed by Rob Barttelbort and Chris Puzinas.

Conclusion

This GPS project tested and taught us a number of things. We learned about one of the "new" technologies in surveying, but we also learned its limitations. To start this project had us work in a Wiki. This was an interesting learning experience for many of us that have not done much with programming. Working within the confines of the Wiki helped with team work and giving all the group members a way to get credit for what they have done. This component also makes it so we can reference this report in the future and it is safe from things such as hard drive failure or loss of ones PC.

Once we started working with the GPS we found is was fairly simple to navigate and didn't take a long time to get the basic functions down. We went back after getting the basic uses down and read that there were more functions to use that made it easier to take readings and use the GPS for different things. <<<<whoever has the manual should put in some of the functions here to expand this>>>>

In terms of results, we did not have a specific assignment to report numbers on but chose to take measurements on what we had done in a previous lab so that we could calculate error. We found that these GPS units are very bad at reporting elevation and hardly work at all if you are surrounded by buildings. These devices work best in open fields so that they can pick up more satellites and get a better idea of location since there is very little to no interference. To get better readings in elevation you just need to pay up and get a better unit. As a result our unit actually produced readings that were decreasing in elevation even as we were increasing in elevation. This is fairly scary since so many people depend on these devices when they are lost. Also, the device gave measurements with accuracy ranging from +/-28ft to +/-46 ft. 28 is a large range, 46 just shows that the device has no clue what your elevation is and might as well use a random number generator. What is worse was that even with these large ranges only one of our measured values was barley within the listed range. On average, the values were off by 123.95 ft. One can only hope that our machine was calibrated wrong or that this was caused by being in the city because with that kind of error this function should not be included.

In conclusion, we all gained from this project weather learning about how to use the Wiki or the GPS unit. None of us would trust elevations as given by this device although it was decent at producing location.